There is interest in forming relatively light-weight magnesium alloy sheet materials into, for example, automotive vehicle body panels. Such panels may be formed from initially flat, sheet metal blanks having nominal dimensions of, e.g., about 1000 mm×1500 mm×1-3 mm. So far, automotive manufacturing engineers have had more experience in forming body panels from aluminum sheet alloys, although magnesium alloys are hot formable at about the same temperature ranges as aluminum alloys and offer further reductions in weight.
At the present time the forming of magnesium alloy sheet metal body panels requires forming the sheet metal at elevated forming temperatures. The specific hot forming practice used depends largely on the complexity of the shape of the panel product; i.e., the severity of the deformation required to be introduced into a magnesium alloy sheet metal blank. Some panel shapes, like engine compartment hoods, may be formed by hot stamping magnesium alloy sheet blanks between facing, complementary heated forming dies without preheating the workpieces. Other, more complex panel shapes may require that the workpieces be preheated for hot blow forming. Magnesium alloy vehicle lift gates and door panels often require hot blow forming temperatures and practices to deform the sheet material into a functional and streamlined panel shape.
Hot blow forming of magnesium sheet metal typically involves heating of a sheet metal blank to approximately 450° C. in a preheat furnace, robotically transferring that sheet to a position between facing dies which are also heated to approximately that same temperature, clamping the periphery of the sheet blank shape between die halves to establish a gas-tight seal, and then applying gas pressure to one side of the heated sheet to blow it into a facing die cavity and against a forming surface to form the desired shape. Then the gas pressure is released, the die is opened, and the formed panel is removed and allowed to cool. Alternatively, in some cases, instead of using a preheat furnace, the sheet may be heated by the hot die before or during forming. In either case, the sheet is typically heated to approximately 450° C., and then held at that temperature for a short time to assure a suitably uniform temperature prior to application of the forming pressure.
In forming by hot stamping, the magnesium alloy sheet material is usually preheated to a temperature below about 350° C. and stamped between heated, complementary forming dies carried on opposing press platens and maintained at a specified forming temperature. Upon press closure, the heated sheet is contacted by at least one die surface which rams and stretches the sheet against a facing surface. As in hot blow forming, the sheet workpiece and the hot stamping tools are at a specified hot stamping temperature before deformation of the workpiece begins.
These hot forming practices are well developed for aluminum sheet alloys and the fully preheated workpieces are formed readily into body panels of complex shape. But the application of such hot forming practices to magnesium sheet alloys has generally been slower and more easily applied to the forming of articles with lower shape complexity.